The razorback sucker population of the lower Colorado River basin is now reduced to scattered individuals in all but Lake Mohave, Arizona-Nevada. That population consists of large, slowly-growing fish, which are proposed to be nearly 30 yr old, presumably having hatched when the reservoir was filling in the early 1950s. The species comprised about 12.5% of all fishes taken by trammel netting in Lake Mohave in the period 1974-1982. No recruitment into the population has been detected in that period, despite repeated observations of spawning and records of fertilized eggs and hatched larvae. Sex ratios, sexual dimorphism and fecundity all indicate the fish to be reproductively capable despite a high incidence of disease. The species is now under artificial propagation and is being reintroduced within its native range. Predation by introduced fishes on early life-history stages of razorback sucker is considered the most important factor in their decline.